This invention relates a liquid crystal display device, and more particularly to a technique useful for a circuit for supplying a video signal voltage to each pixel.
An active-matrix type liquid crystal display device having an active element for each pixel (for example, a thin film transistor) and switching the active elements has been used widely as a display device of a notebook personal computer or the like.
Among the active-matrix type liquid crystal display devices, a TFT (Thin Film Transistor) type liquid crystal display module has been known. In the TFT type liquid crystal display module, since a video signal voltage (a gray scale voltage) is applied to a pixel electrode via a thin film transistor (TFT), the TFT type liquid crystal display module is free from crosstalk between pixels, and therefore that the TFT type liquid crystal display module is capable of providing a multi-gray scale display without using a special driving method for preventing the crosstalk, unlike a simple matrix type liquid crystal display device.
However, when a D/A conversion which selects a gray-scale voltage corresponding to a display data in digital form is used for supplying the gray-scale voltage to a pixel electrode, problems arise that, as the number of gray scales increases, the number of bits representing a display data increases, and consequently, the scale of circuits becomes large and further the speed of operation of the circuits becomes insufficient. Further, especially in liquid crystal display devices of the driving-circuit-integrated type having driving circuits and a display section fabricated on the same substrate, the above problems are serious because they increase the area of the driving circuit section other than the useful display area.
There is a tendency for output signals from video equipment to be supplied in digital signals rather than in analog signals, and therefore there is a demand for a driving method for converting digital signals into multi-gray-scale video signal voltages by inputting digital signals into the liquid crystal display device and using a driving circuit fabricated on a liquid crystal display panel, in the liquid crystal display devices of the driving-circuit-integrated type also.
As a driving method for applying multi-gray-scale video signal voltages to each pixel so that a multi-gray-scale display can be produced by using digital signal input in the active matrix type liquid crystal display device, one method of driving is known which is disclosed in Japanese Patent Application Laid-open No. Hei 5-35200 (corresponding to U.S. Pat. No. 5,337,070).
In the method disclosed in Japanese Patent Application Laid-open No. Hei 5-35200, 2m voltage bus lines are provided, and each of gray scale voltages provided from the 2m voltage bus lines varies in a staircase fashion having 2k steps during one horizontal scanning period corresponding to one horizontal scanning line.
One of the above-mentioned 2m voltage bus lines is selected based on the high-order m bits of an n-bit display data, one of the voltage levels is selected based on the lower-order k (k=n−m) bits of the n-bit display data, from the gray scale voltage varying in the staircase fashion on the selected voltage bus line, and the selected voltage level is applied to a pixel electrode of a pixel.
For example, assume a case in which the display data is 3 bits (n=3), m=1, and k=2. Two voltage bus lines are provided and each voltage bus line is supplied with a gray scale voltage varying in a staircase fashion having four steps during one horizontal scanning period. A gray scale voltage on one of two voltage bus lines is selected based on the high-order 1 bit of the 3-bit display data, one voltage level is selected from the gray scale voltage varying in the staircase fashion having four steps on the selected voltage bus line, based on the lower-order 2 bits of the 3-bit display data, and the selected voltage level is applied to the pixel electrode of a pixel.
According to the driving method described in the above-mentioned Japanese Patent Application Laid-open No. Hei 5-35200, the operating speed of the circuit for applying a video signal voltage on each pixel can be reduced, variations in the video signal voltages caused by the D/A conversion are reduced over the entire display area, and the number of voltage bus lines can be reduced.
However, when the number of the gray-scale levels are increased to improve display quality, the scale of a selector circuit for selecting one of voltage levels varying in a staircase fashion is made larger, and an area occupied by the selector circuit becomes so large in incorporating it into the liquid crystal display panel, and consequently, a problem arises in that the liquid crystal display panel becomes large-sized. As a liquid crystal display device solving the above-problem, a technique for reducing the width of the selector circuit is known which is disclosed in Japanese Patent Application Laid-open No. 2000-194330.